Method of controlling drying of timber in a closed room

ABSTRACT

A method of controlling the drying of timber which comprises placing the timber in a closed drying chamber, sensing its humidity and comparing it to a reference value, initiating ventilation of the chamber when a certain difference relationship exists between the sensed humidity and said reference value, discontinuing ventilation when another difference relationship exists, effecting further ventilation when a certain difference relationship exists between the sensed humidity and said reference value, determining the rate at which actual humidity in the drying chamber changes per unit of time during the interval between the cessation of ventilation and initiation of ventilation, and changing the reference value when the rate determined falls outside a certain range.

United States Patent [191 L'dvgren [4 1 July 29, 1975 [75] Inventor:

[73] Assignee: Alfred Liivgren Byggnads AB,

Skelleftea, Sweden [22] Filed: Nov. 5, 1973 [21] Appl. No.: 413,131

Anders L'dvgren, Skelleftea, Sweden 3,744,144 7/1973 Weis 34/l6.5

Primary Examiner-John J. Camby Attorney, Agent, or FirmFred Philpitt [57] ABSTRACT A method of controlling the drying of timber which comprises placing the timber in a closed drying chamber, sensing its humidity and comparing it to a reference value, initiating ventilation of the chamber when a certain difference relationship exists between the sensed humidity and said reference value, discontinuing ventilation when another difference relationship exists, effecting further ventilation when a certain difference relationship exists between the sensed humidity and said reference value, determining the rate at which actual humidity in the drying chamber changes per unit of time during the interval between the cessation of ventilation and initiation of ventilation, and changing the reference value when the rate determined falls outside a certain range.

6 Claims, 3 Drawing Figures PATENTED JUL 2 9 i975 SHEET PATENTED JUL 2 9 I975 SHEEE NGE METHOD OF CONTROLLING DRYING OF TIMBER IN A CLOSED ROOM Timber contains, as known, moisture and must in many cases be dried after sawing for being applicable. At timber drying one utilizes the known fact that the moisture within the timber tends to assume a state of equilibrium, which depends on or corresponds to the condition of the ambient air, irrespective of whether the drying takes place in open-air timber yards by action of solar heat and wind or artificially in drying houses. In the firstmentioned case, one cannot, for obvious reasons, affect the condition of the ambient air, but the open-air drying depends to a high degree on the weather and season, which implies a long drying time. Therefore, artificial drying in closed rooms is being applied to an increasing extent, in which rooms the condition of the air surrounding the timber can be influenced and changed without great difficulty, a.o. by using heated air as control medium.

The artificial or controlled drying of sawn timber usually takes place in travelling dryers, in which bundles of timber of substantially the same dimensions are advanced in steps through zones of different air condition, or in chamber dryers, in which the timber is arranged stationary and the condition of the surrounding air is changed, for example by injection of heated air until the drying of the timber is completed. Irrespective of whether this change in condition is brought about by constant or discontinued injection of heated air, or in another way, it is to be controlled so that the timber in the drying chamber is not subjected to high internal shrinkage stresses, which a.o. would result in crack formation as, for example, in the case of too rapid drying, or to fungus attacks (discolourization or mould).

The drying process in a chamber dryer heretofore was controlled either by program control according to a predetermined time and climate procedure, i.e. by predetermining the allowable drying time and accordingly controlling the drying climate, or by controlling the dehumidifrcation obtained in the timber in the chamber, i.e. by continuously measuring the moisture content of the timber and proceeding with the drying until the timber shows a predetermined moisture content. Neither of these methods, however, has proved satisfactory. In the firstmentioned case, a great number of different programs is required, and one must know in advance the optimum drying process for the timber in question, so that the timber will be dried in the way intended. In the second case, a relatively rapid drying implying the risk of crack formation is obtained at the beginning when the moisture content in the timber is relatively high, and a relatively slow, inefficient drying, which results in a substantially longer total drying time, is obtained toward the end of the drying process when the moisture content in the timber is relatively low.

A further disadvantage of the known methods of controlling timber drying in closed rooms or chambers is that the methods cannot be applied to the drying of wood species with a high cracking tendency, for example birch.

The object of the present invention, therefore, is to develop a method of controlling the drying of timber in a closed chamber or another space, for example in a chamber dryer, which does not involve the disadvantages of the known methods and renders possible an optimum drying rate with respect to the design of the drying chamber and the nature of the timber, and a total drying time in full adaptation to the circumstances, i.e. the amount of water bound in the timber, the timber density, structure a.s.o., and which method thereby renders it possible to maintain the highest possible relative humidity in the chamber and thereby eliminates the requirement of soaking, i.e. wetting the timber during drying, which is necessary for effecting a rapid drying with the known methods of controlling timber drying. Due to the high humidity during the drying, it is possible according to the invention to dry relatively rapidly also such timber, which has a high cracking tendency, for example leafwood. The invention, further, relates to a device for carrying out the method. This is achieved in that the method and device according to the invention show the characterizing features defined in the claims.

The invention is described in greater detail in the following, with reference to the accompanying drawings. in which FIG. 1 shows a block diagram for an embodiment of the device according to the invention, and

FIG. 2 shows two curves a and b, which illustrate the relation between relative humidity in the drying chamber and the time at the method and device according to the invention and, respectively, the relation between the temperature of the air surrounding the timber and the time during a drying process.

At the embodiment according to the invention described in the following it is presupposed that a suitable drying temperature in the drying chamber is achieved relatively rapidly and maintained substantially constant until the moisture content in the timber is reduced to a predetermined value, as shown in a schematic manner by the curve b in FIG. 2. The said predetermined value, which at a given temperature corresponds to a definite value of the relative humidity in the drying chamber, is fed into a drying program, which also includes the upper allowable value for the relative humidity in the chamber, for example above which value a continuous ventilation of the chamber always will take place.

A transmitter 1 for relative humidity senses the relative humidity in the drying chamber shown in the drawings and emits a signal, which is proportional to the read relative humidity, to an amplifier 2, which has an output voltage proportional to the relative humidity. The amplifier is connected to a comparator 3 with an upper and lower hysteresis level at its input for comparing the actual value, i.e. the relative humidity in the chamber, with an upwardly and downwardly adjustable nominal value, which thus must not be greater than the value for the highest allowable relative humidity in the drying chamber. When the actual value exceeds the nominal value valid at the moment in question plus the difference between said nominal value and the upper hysteresis level, the comparator 3 effects ventilation of the drying chamber, i.e. the relative humidity is lowered by one or more fans 8, which ventilation continues until the humidity in the drying chamber falls below the nominal value valid at the moment in question minus the difference between said nominal value and the lower hysteresis level of the comparator. When this condition has been achieved, the ventilation stops and thereby rises due to evaporation from the timber the humidity in the drying chamber until the actual value entering the comparator again amounts to the upper hysteresis level of the comparator, i.e. to the nominal value valid at the moment in question plus the difference between said nominal value and the upper hysteresis level. The time passing between each ventilation is measured and supervised by a programmable chronometer 4 with two outputs x and, respectively, y and with data input means 5 indicating an upper and a lower limit for the time between each ventilation. When this predetermined time interval is exceeded, i.e. when it takes a longer time to achieve the humidity in the chamber corresponding to the valid nominal value than indicated by the programmed time interval, the chronometer 4 emits a pulse, for example a one, at the output x to a programmable nominal value switch 6, and in a corresponding manner a pulse is emitted, for example a one, at the output y when said time interval is fallen below. When the time between each ventilation is kept within the programmed interval, the chronometer 4 does not emit a pulse at any of its outputs x, y.

The programmed actual value switch 6 has an input x,, which is connected to the output x of the chronometer, and an input y which is connected to the output y of the chronometer, and is programmed so as at pulses at the input x, to decrease in steps the nominal value, i.e. the reference voltage, by a predetermined degree, for example by 5% per step, and, respectively, at pulses at the input y, to increase the nominal value by a predetermined degree, for example by 5% per step. The comparator 3 is informed on this new nominal value and thereafter compares the actual value with the new nominal value a.s.o. until the program has arrived at the lowest nominal value, which can be set separately, for example in the setting means designated by 7, and corresponds to the predetermined final value of the moisture content in the timber.

According to the described embodiment of the invention, thus, the diffusion rate is measured and controlled by studying the increase in the relative or absolute humidity of the air per time unit. This is effected by measuring the time from the stop of the ventilation to its new start, i.e. the time 2 (see the curve A in FIG. 2) which it takes for the actual value voltage proportional to the relative humidity in the chamber to rise from the lower hysteresis level of the comparator 3 to its upper hysteresis level.

As the timber in the drying chamber dries, its capacity of maintaining the relative humidity in the drying chamber decreases, and ventilation need not be carried out as often, i.e. the time passing between each ventilation increases. When this time exceeds a certain value t the programmed reference voltage or the nominal value is stepped down one step, which may correspond to 5% as indicated above, but of course also other percentages can be taken into consideration. In other words: a new nominal value for the relative humidity in the drying chamber is obtained and thereafter maintained as long as possible in the chamber. The program is, as mentioned, reversible so that at too high an increase in the diffusion rate the program steps back one or more steps, thereby increasing the nominal value for the relative humidity and, thus, decreasing the diffusion rate. Hereby a balanced diffusion rate is obtained, and consequently a drying rate, which is at optimum with respect to the design of the drying chamber and the nature of the timber. The method according to the invention further renders a total drying time, which is fully adjusted to the prevailing circumstances as regards the amount of water bound 1' the timber, the timber sity, structure a.s.o.

The present invention is not restricted to what is described above and shown in the drawings, but may be altered and modified in many different ways within the scope of the claims. It is possible, thus, to measure the, increase of the relative humidity within the chamber for a certain definite period of time instead of for the time it takes for the relative humidity to rise a predetermined number of percents.

What I claim is:

l. A method of controlling the drying of timber which comprises a. placing the timber in a closed drying chamber,

sensing the humidity in the closed chamber and emitting an electronic signal in proportion to the thus sensed humidity,

b. feeding the signal generated in step (a) to a comparator means having an upper and lower hysterisis level at its input and comparing the signal generated in step (a), which represents the actual humidity in the drying chamber, with an upwardly and downwardly adjustable nominal value for humidity which must not be greater than the value for the highest allowable humidity in the drying chamber, c. initiating ventilation of the drying chamber when the actual value of the humidity exceeds the nominal value valid at the moment in question plus the difference between said nominal value and the upper hysterisis level of said comparator means,

d. discontinuing ventilation of the drying chamber when the actual humidity value in the drying chamber falls below the nominal value valid at the moment in question minus the difference between said nominal value and the lower hysterisis level of the comparator means,

e. effecting further ventilation of the drying chamber when the actual value of the actual humidity in the drying chamber exceeds the nominal value at the moment in question plus the difference between said nominal value and the upper hysterisis level of said comparator means,

f. repeating steps (d) and (e) any desired number of times,

g. measuring the time elapsed between each cessation of ventilation and each subsequent initiation of ventilation,

h. changing the aforesaid nominal value when the time measured in step (g) falls outside a certain time interval.

2. A method of controlling the drying of timber which comprises a. placing the timber in a closed drying chamber,

b. sensing the humidity in the closed chamber,

c. comparing the sensed humidity to a reference value,

(1. initiating ventilation of the drying chamber when a certain difference relationship exists between the sensed humidity and said reference value,

e. discontinuing ventilation of the drying chamber when another difference relationship exists between the sensed humidity and said reference value,

f. effecting further ventilation of the drying chamber when a certain difference relationship exists between the sensed humidity and said reference value,

g. repeating steps (e) and (f) any desired number of times,

h. determining the rate at which actual humidity in the drying chamber changes per unit of time during the interval between the cessation of ventilation and initiation of ventilation, and

. changing the reference value set forth in step (0) when the rate determined in step (h) falls outside a certain range.

3. A method of controlling the drying of timber which comprises a. placing the timber in a closed drying chamber,

b. ventilating said drying chamber in a manner which is dependent upon the relationship between the humidity in the chamber and a reference value.

c. determining the rate of humidity change per unit of time during at least part of the time the timber is in the drying chamber, and

d. varying the magnitude of the reference value used in step (b) when the rate determined in step (c) falls outside a certain range.

4. An arrangement for drying timber comprising a. a drying chamber adapted to contain timber,

b. a ventilator for ventilating said drying chamber,

c. sensor means for sensing the humidity in the drying chamber,

d. comparator means for comparing the value sensed by said sensor means with a reference value,

e. means for actuating said ventilator in accordance with the output of said comparator means,

f. means for determining the rate at which the humidity of the atmosphere changes per unit of time in the drying chamber, and

g. program means for varying the magnitude of the reference value utilized by the comparator means when the rate determined by the means of (f) falls outside a certain range.

5. An arrangement according to claim 4 wherein means (f) comprises a chronometer which indicates the upper and lower limit for the time between the stopping and starting of the ventilator and is operatively connected to a reference value switch in the program means, which in turn is operatively connected to the comparator means so as to be able to transmit thereto differing reference values.

6. An arrangement according to claim 4 wherein the program means includes a means for measuring the change in humidity for a predetermined period of time and a means for actuating a reference value switch cacomparator means. 

1. A method of controlling the drying of timber which comprises a. placing the timber in a closed drying chamber, sensing the humidity in the closed chamber and emitting an electronic signal in proportion to the thus sensed humidity, b. feeding the signal generated in step (a) to a comparator means having an upper and lower hysterisis level at its input and comparing the signal generated in step (a), which represents the actual humidity in the drying chamber, with an upwardly and downwardly adjustable nominal value for humidity which must not be greater than the value for the highest allowable humidity in the drying chamber, c. initiating ventilation of the drying chamber when the actual value of the humidity exceeds the nominal value valid at the moment in question plus the difference between said nominal value and the upper hysterisis level of said comparator means, d. discontinuing ventilation of the drying chamber when the actual humidity value in the drying chamber falls below the nominal value valid at the moment in question minus the difference between said nominal value and the lower hysterisis level of the comparator means, e. effecting further ventilation of the drying chamber when the actual value of the actual humidity in the drying chamber exceeds the nominal value at the moment in question plus the difference between said nominal value and the upper hysterisis level of said comparator means, f. repeating steps (d) and (e) any desired number of times, g. measuring the time elapsed between each cessation of ventilation and each subsequent initiation of ventilation, h. changing the aforesaid nominal value when the time measured in step (g) falls outside a certain time interval.
 2. A method of controlling the drying of timber which comprises a. placing the timber in a closed drying chamber, b. sensing the humidity in the closed chamber, c. comparing the sensed humidity to a reference value, d. initiating ventilation of the drying chamber when a certain difference relationship exists between the sensed humidity and said reference value, e. discontinuing ventilation of the drying chamber when another difference relationship exists between the sensed humidity and said reference value, f. effecting further ventilation of the drying chamber when a certain difference relationship exists between the sensed humidity and said reference value, g. repeating steps (e) and (f) any desired number of times, h. determining the rate at which actual humidity in the drying chamber changes per unit of time during the interval between the cessation of ventilation and initiation of ventilation, and i. changing the reference value set forth in step (c) when the rate determined in step (h) falls outside a certain range.
 3. A method of controlling the drying of timber which comprises a. placing the timber in a closed drying chamber, b. ventilating said drying chamber in a manner which is dependent upon the relationship between the humidity in the chamber and a reference value, c. determining the rate of humidity change per unit of time during at least part of the time the timber is in the drying chamber, and d. varying the magnitude of the reference value used in step (b) when the rate determined in step (c) falls outside a certain range.
 4. An arrangement for drying timber comprising a. a drying chamber adapted to contain timber, b. a ventilator for ventilating said drying chamber, c. sensor means for sensing the humidity in the drying chamber, d. comparator means for comparing the value sensed by said sensor means witH a reference value, e. means for actuating said ventilator in accordance with the output of said comparator means, f. means for determining the rate at which the humidity of the atmosphere changes per unit of time in the drying chamber, and g. program means for varying the magnitude of the reference value utilized by the comparator means when the rate determined by the means of (f) falls outside a certain range.
 5. An arrangement according to claim 4 wherein means (f) comprises a chronometer which indicates the upper and lower limit for the time between the stopping and starting of the ventilator and is operatively connected to a reference value switch in the program means, which in turn is operatively connected to the comparator means so as to be able to transmit thereto differing reference values.
 6. An arrangement according to claim 4 wherein the program means includes a means for measuring the change in humidity for a predetermined period of time and a means for actuating a reference value switch capable of establishing a different reference value for said comparator means. 